Aaaagh, it's driving me crazy! My yamaha is massively sharp on two strings at 7,9 and 12 and flat on 1 string. I know how to fix the flat (using a diamond nail file etc) but not the sharps, can it be done with a thicker saddle? Effectively shortening the string length by a mm and then filing it to correct the other strings
Solved! After much trial and fail with the saddle I found a brilliant luthier at my local shop who gave me (and yes I do mean gave me) a set of d'addario Pro arte strings, the D which with d'addario classic strings was a semi tone sharp at 12th fret is now perfect as are all the strings. Who would believe that strings could make SO much difference! Note to other Yamaha C40 owners:They don't love D'Addario classic strings! And yes I went back to the shop and thanked and paid the brilliant Luthier Richard.
You can adjust the intonation either sharper or flatter by filing on the back or front of the saddle.
To adjust flatter, file the front side of the crown of the saddle to move the point of contact back and lengthen the sounding portion of the string.
To adjust sharper, file the back side of the crown of the saddle to move the point of contact forward and shorten the sounding portion of the string.
(Be sure to mark the saddle when you remove it so you can reinstall it with the same orientation.)